Titanium carbide as a friction and wear modifier in friction materials

ABSTRACT

Methods of making a carbon-carbon composite preforms, particularly suitable as brake discs in aircraft landing systems, by combining titanium carbide particles ranging in size from 0.01 to 10 microns in diameter, resinous binder, and carbon fibers or carbon fiber precursors in a mold, and subsequently subjecting the combined components to pressure and heat to carbonize the resinous binder by methods, thereby providing the carbon-carbon composite preform having particulate titanium carbide uniformly distributed throughout its mass. Prior to combining the titanium carbide and the binder with the fibers in this process, the particulate titanium carbide may be mixed with liquid binder, the resulting TiC/binder mixture may then solidified, and the resulting solid TiC/binder mixture may be ground into a fine powder for use in the process. Also, compositions for preparing a carbon-carbon composite friction materials, and methods of improving wear and dynamic stability in a carbon-carbon composite brake discs.

PRIORITY

This application claims priority under 35 U.S.C. § 119(e) to U.S.Provisional Application No. 60/558,111 for “TITANIUM CARBIDE-ENHANCEDCARBON-CARBON BRAKE COMPOSITES,” filed on Apr. 1, 2004, the entirecontents of which are hereby expressly incorporated by reference.

FIELD OF THE INVENTION

The present invention is in the field of carbon-carbon compositematerials. In particular, this invention contemplates carbon-carboncomposite materials that are useful as friction components, for instancein aircraft landing system brake assemblies.

BACKGROUND OF THE INVENTION

Many aircraft brakes are made of carbon-carbon composite materials.Aircraft brakes are subjected to high temperatures, which may change theproperties of the carbon and the friction surface and lead to variationin the friction performance of the brake. Other factors that mayincrease variation in friction performance of carbon-carbon compositematerials are variation in the carbon microstructure, variation in thefiber/matrix ratio, and differences in the energy levels of the previousstop or stops performed by the brake. Also, a desirable property of acarbon-carbon friction material is a low wear rate. The presentinvention provides a method of improving the wear rate of acarbon-carbon composite friction material. The present invention at thesame time provides a carbon-carbon composite friction material which hasa very stable friction performance.

SUMMARY OF THE INVENTION

It has been discovered that titanium carbide, when uniformly distributedinto carbon-carbon composite preforms in the form of particles ofparticular particle size, provides the preforms with beneficiallymodified friction and wear properties. Carbon-carbon composite brakesmade in accordance with the present invention have improved wear ratesand stable, consistent friction performance.

Accordingly, this invention provides a method of improving wear anddynamic stability in a carbon-carbon composite brake disc. This methodembodiment of the present invention involves manufacturing thecarbon-carbon composite brake discs from preforms comprisingcarbon-containing fibers, resin binder, and titanium carbide particleshaving a particle diameter in the range of from 0.01 to 10 microns. Inaccordance with the present invention, the dynamic stability of thebrakes manufactured is characterized by a brake effectiveness μ of lessthan 0.200, preferably less than 0.100, wherein μ is defined by theformula$\mu = \frac{T_{Average}}{\left( {p_{Average} - {bprt}} \right)\left( {2N_{R}} \right)A_{p}R_{m}}$in which T_(Average), P_(Average), bprt, 2N_(R), A_(P), and R_(m) are asdefined hereinbelow.

Another embodiment of the present invention is a carbon-carbon compositebrake disc preform comprising carbon fibers and resin binder, whereinthe preform has particulate titanium carbide uniformly distributedthroughout its mass. The particles of titanium carbide in thisembodiment generally range in size from 0.01 to 10 microns in diameter.Preferably the particles have an average particle size of 1-2 micronsand an apparent porosity in the range 0.5-0.6. A particularly preferredparticulate titanium carbide has an average particle size of 1.4 micronsand an apparent porosity of 0.545.

Still another embodiment of this invention is a method of making acarbon-carbon composite preform. In this method, one combines, e.g. 0.1to 15 parts by weight titanium carbide particles ranging in size from0.01 to 10 microns in diameter, 20 to 85 parts by weight resinousbinder, and 20 to 80 parts by weight carbon fibers or carbon fiberprecursors in a mold. The titanium carbide particles used in thisprocess preferably have an average particle size of 1-2 microns and anapparent porosity in the range 0.5-0.6. Subsequently, one subjects thecombined components to pressure and heat to carbonize the resinousbinder by methods that are in general familiar to those skilled in theart, thereby providing a carbon-carbon composite preform havingparticulate titanium carbide uniformly distributed throughout its mass.In this method, the mold is most preferably one that is configured toprovide a preform in the shape of an aircraft landing system brake disc.Prior to combining the titanium carbide and the binder with the fibersin this process, the particulate titanium carbide may be mixed withliquid binder, the resulting TiC/binder mixture may then be solidified,and the resulting solid TiC/binder mixture may be ground into a finepowder for use in the process.

Yet another embodiment of the present invention is a composition forpreparing a carbon-carbon composite friction material. Thiscompositional embodiment is made up of carbon fibers or carbon fiberprecursors, powdered or liquid resin binder, and titanium carbideparticles ranging in size from 0.01 to 10 microns in diameter. In thiscomposition, the carbon fiber or carbon fiber precursors may constitute15-80 weight-% of the composition, the powdered or liquid resin bindermay constitute 20-85 weight-% of the composition, and the titaniumcarbide particles may constitute 0.1-15 weight-% of the composition.

DETAILED DESCRIPTION OF THE INVENTION

Titanium carbide powder may be included in a composite material duringthe manufacturing stage, along with the carbon fibers and matrixmaterial. In accordance with this invention, the titanium carbideparticles have diameters in the range 0.01 to 10 microns. Particle sizeof titanium carbide powders may be determined in accordance with theprocedures described in ASTM B-330-02. It has been found that, althoughexcellent friction performance can be achieved with titanium carbideparticles larger than 10 microns in diameter, the wear rate ofcarbon-carbon composites made with such particles increases to levelsthat are generally unacceptable. More preferably, the particles are inthe range 0.5 to 5 microns in diameter. A specific example of aparticulate titanium carbide that may be used in this invention is Grade2049 TiC from Pacific Particulate Materials Ltd. of Port Coquitlam,B.C., Canada.

The carbon-carbon composite preforms of this invention include from 15to 80 weight-%, preferably from 40 to 80 weight-%, more preferably from50 to 65 weight-%, carbon fiber, from 20 to 85 weight-%, preferably from20 to 65 weight-%, more preferably from 30 to 45 weight-%, binder, andfrom 0.1 to 15 weight-%, preferably from 1 to 15 weight-%, morepreferably from 2 to 8 weight-%, of the titanium carbide particles. Itis important that the titanium carbide (or titanium component) bedispersed or uniformly distributed throughout the entire carbon-carboncomposite that will be used as friction material. This will ensureconsistent friction performance throughout the life of the brake.

Those skilled in the art are familiar with many methods of makingcarbon-carbon friction materials. In all cases, carbon fibers—or carbonfiber precursors, such as pitch fibers or polyacrylonitrile (PAN)fibers—are used to provide architecture and strength to the composite.The fibers may be of random orientation or the fibers may have awell-defined architecture, obtained e.g. by a controlled spraying ofchopped fibers into a mold. Fibers ranging in length from 1 to 30 mm arenormally employed in this invention. However, fibers of other lengthsmay be used. While it is often convenient to make use of “chopped”fibers, the only requisite of the form of the fibers is that they permitthe titanium carbide particles to be uniformly distributed in theregions of the preform that will function as friction material.Accordingly, other sorts of fibers, including “continuous” fibers, maybe employed in this invention. One specific example of a carbon fiberthat can be used in the present invention is Zoltek 48K carbon fiber,available from Zoltek Corporation of St. Louis, Mo.

A carbon-bearing resinous binder, such as a pitch-based binder or aphenolic resin binder, is also a significant part of the presentcarbon-carbon composite friction materials. Those skilled in the art ofmaking carbon-carbon composite friction materials are familiar with awide variety of suitable pitch-based, phenolic, and similar resinbinders. The resin binder is carbonized during processing of theprecursor mixture to prepare a preform which has a carbon matrix bondedto the reinforcement fibers.

In accordance with the present invention, titanium carbide particles aredispersed or uniformly distributed throughout the entire carbon-carboncomposite, e.g., by subjecting a precursor mixture composition to mixingin a Hobart blender. One approach for ensuring good distribution is tomix the TiC powder with hot liquid binder, solidify the mixture bycooling, and then grinding the solid TiC/binder mixture into a finepowder. This coats the titanium carbide particles with binder andensures that distribution of the titanium carbide in the preform isquite uniform, because when the preform is heated and pressed, theliquid flows and fills voids in the fibrous matrix of the preform.Uniform distribution of titanium carbide throughout the carbon-carboncomposite friction preform ensures that the friction film generated bywear is constant through the life of a brake made from the carbon-carboncomposite preform of this invention.

EXAMPLES Example 1

Five parts by weight of particulate titanium carbide, having an averageparticle size of 1.38 microns, an apparent porosity of 0.545, and adensity of 4.93 g/cc, is added to 35 parts by weight liquidthermoplastic phenolic resin binder. To ensure adequate mixing, thetitanium carbide powder and the phenolic resin powder are passed throughan extruder and then re-ground into a powder. Subsequently, 60 parts byweight chopped carbon fibers are added to the powder. The mixture isthoroughly blended and then decanted into an annular mold, where it ispressed and heated to set the binder. Then the molded composition ischarred to produce a carbon-carbon composite brake disc preform. Thepreform is densified by conventional CVI/CVD processing and machined toprepare a brake disc.

Example 2

Two hundred grams of particulate titanium carbide (nominal particle size1.4 microns) is added to 1400 grams of liquid thermoplastic phenolicresin binder to provide 1600 grams of binder. To ensure adequate mixing,the titanium carbide powder and the phenolic resin powder are passedthrough an extruder and then re-ground into a powder. Separately, 2060grams of carbon fibers (Zoltec 48K, having a density of 1.78 g/cc) areprovided. The fibers and the powder mixture are decanted into an annularmold, having the form of an aircraft landing system brake disc (stator),as follows. The binder powder is divided into 39 approximately equallots and the fibers are divided into 38 approximately equal lots. Thebinder powder lots (containing TiC) and the fiber lots are depositedalternatively into the mold. Once all 3660 grams of material have beendeposited into the mold in this way, the combined materials are pressedand heated to set the binder and consolidate the preform. Theconsolidation step includes pressurization to 200 psi for 50 minutes.The temperature is maintained at a level of 215° F. for 45 minutesduring the consolidation step. The molded composition is then charred toproduce a carbon-carbon composite brake disc preform. The preform isdensified by conventional CVD processing and machined to prepare a brakedisc having a thickness of 2.86 cm.

Example 3

Brake discs manufactured by conventional procedures (runs A-D) and brakediscs made in accordance with the present invention (run E) were testedon a subscale dynamometer, with the following results: Wear per Surfaceper Stop Run (10⁻⁴ inches) Effectiveness A Production Baseline Material0.1183 0.291 B Production Baseline Material 0.1333 0.351 C 70% Fiber/30%Phenolic 0.1833 0.250 D Pitch Binder 0.1033 0.321 E 60% Fiber/40%Phenolic/TiC 0.0933 0.056

It can be seen that Run E, an embodiment of the present invention,demonstrated less wear. It also significantly lowered “effectiveness” ascompared to Comparative Runs A-D. Reduced wear rates provide a cleareconomic advantage. Lower “effectiveness” increases the dynamicstability of aircraft braking systems.

Brake effectiveness is defined as a non-dimensional quantity relatingthe compressive (normal) force to the braking torque. In other words, itis a rotating machinery equivalent to the coefficient of friction. Inthe aircraft brake industry, the brake effectiveness is expressed as:$\mu = \frac{T_{Average}}{\left( {p_{Average} - {bprt}} \right)\left( {2N_{R}} \right)A_{p}R_{m}}$where:

-   -   a. T_(Average)—Average torque generated by the brake    -   b. P_(Average)—Average brake hydraulic fluid pressure    -   c. bprt—Brake pressure rotors tight, lowest pressure at which        brake generates torque    -   d. 2N_(R)—Number of friction surfaces (twice the number of        rotors N_(R))    -   e. A_(P)—Total hydraulic fluid piston surface area    -   f. R_(m)—Mean brake radius.

With respect to the effectiveness, aircraft brake friction materials aredesigned to satisfy four main functional goals: a) low effectivenessduring most landing and taxi conditions; b) high effectiveness during aRejected Take-Off (RTO) stop; c) low within stop variability (constanteffectiveness values during the stop); and d) low average effectivenessvariability between stops at different operating conditions. Loweffectiveness during landing and taxi conditions is important formaintaining vibration free brake operation, which helps to reduceunscheduled brake removals and leads to increased profitability for thebrake operator. An additional financial benefit usually associated withlow effectiveness rates during landing and taxi is low wear rate (longbrake life)

High effectiveness during RTO is a necessary condition for the safeoperation of the aircraft during emergency braking conditions. Meetingthe minimum RTO stopping distance specification is a mandatoryrequirement for brake qualification on the airplane.

Low within stop variability requirement is necessary for smooth brakingoperation preferred by airplane crew and passengers and also to ensurethat the peak torque generated by the brake does not exceed thestructural limits of the landing gear.

Low effectiveness variability under varying operating conditions isnecessary to ensure good compatibility of the brake performance with theBrake Control System. Brake Control Systems can be tuned much easier tobrakes that are predictable under various temperature, velocity, andpressure conditions.

1. A method of improving wear and dynamic stability in a carbon-carbon composite brake disc, which method comprises manufacturing said carbon-carbon composite brake disc from a preform comprising carbon-containing fibers, resin binder, and titanium carbide particles having a particle diameter in the range of from 0.01 to 10 microns.
 2. The method of claim 1, wherein the dynamic stability is characterized by a brake effectiveness μ of less than 0.200, wherein $\mu = \frac{T_{Average}}{\left( {p_{Average} - {bprt}} \right)\left( {2N_{R}} \right)A_{p}R_{m}}$ where: T_(Average—Average torque generated by the brake;) P_(Average—Average brake hydraulic fluid pressure;) bprt—Brake pressure rotors tight, lowest pressure at which brake generates torque; 2N_(R)—Number of friction surfaces (twice the number of rotors N_(R)); A_(Ppl —Total hydraulic fluid piston surface area; and) R_(m)—Mean brake radius.
 3. The method of claim 2, wherein the brake effectiveness μ is less than
 0. 100.
 4. The method of claim 1, wherein said preform comprises from 15 to 80 weight-% carbon fiber, from 20 to 85 weight-% binder, and from 0.1 to 15 weight-% titanium carbide particles.
 5. The method of claim 4, wherein said preform comprises from 50 to 65 weight-% carbon fiber, from 30 to 45 weight-% binder, and from 2 to 8 weight-% titanium carbide particles, said titanium carbide particles ranging in size from 0.5 to 5 microns in diameter.
 6. A carbon-carbon composite brake disc preform comprising carbon fibers and resin binder, said preform having particulate titanium carbide uniformly distributed throughout its mass, wherein the particles of titanium carbide range in size from 0.01 to 10 microns in diameter.
 7. The preform of claim 6, wherein said particulate titanium carbide has an average particle size of 1-2 microns and an apparent porosity in the range 0.5-0.6.
 8. The preform of claim 7, wherein said particulate titanium carbide has an average particle size of 1.4 microns and an apparent porosity of 0.545.
 9. The preform of claim 6, comprising from 15 to 80 weight-% carbon fiber, from 20 to 85 weight-% binder, and from 0.1 to 15 weight-% titanium carbide particles.
 10. The preform of claim 9, comprising from 40 to 80 weight-% carbon fiber, from 20 to 65 weight-% binder, and from 1 to 15 weight-% titanium carbide particles.
 11. The preform of claim 10, comprising from 50 to 65 weight-% carbon fiber, from 30 to 45 weight-% binder, and from 2 to 8 weight-% titanium carbide particles, wherein said titanium carbide particles range in size from 0.5 to 5 microns in diameter.
 12. The preform of claim 11, comprising about 56 weight-% carbon fiber, about 38 weight-% binder, and about 5 weight-% titanium carbide particles, wherein said titanium carbide particles have an average particle size of about 1.4 microns.
 13. A method of making a carbon-carbon composite preform, wherein said method includes: combining titanium carbide particles ranging in size from 0.01 to 10 microns in diameter, resinous binder, and carbon fibers or carbon fiber precursors in a mold, and subjecting the combined components to pressure and heat to carbonize the resinous binder, thereby providing a carbon-carbon composite preform having particulate titanium carbide uniformly distributed throughout its mass.
 14. The method of claim 13, wherein the mold is selected to provide a preform configured as an aircraft landing system brake disc.
 15. The method of claim 13, wherein, prior to combination of the titanium carbide and the binder with the fibers, the particulate titanium carbide is mixed with liquid binder, the resulting TiC/binder mixture is solidified, and the resulting solid TiC/binder mixture is ground into a fine powder.
 16. The method of claim 13, wherein the titanium carbide particles have an average particle size of 1-2 microns and an apparent porosity in the range 0.5-0.6.
 17. The method of claim 13, which comprises combining 0.1 to 15 weight-% titanium carbide particles and 20 to 85 weight-% resinous binder with 15 to 80 weight-% carbon fiber or carbon fiber precursors.
 18. The method of claim 17, which comprises combining from 2 to 8 weight-% titanium carbide particles ranging in size from 0.5 to 5 microns in diameter with from 30 to 45 weight-% binder and from 50 to 65 weight-% carbon fiber.
 19. A composition for preparing a carbon-carbon composite friction material, said composition comprising carbon fibers or carbon fiber precursors, powdered or liquid resin binder, and titanium carbide particles ranging in size from 0.01 to 10 microns in diameter.
 20. The composition of claim 19, comprising 15 to 80 weight-% carbon fiber or carbon fiber precursors, 20 to 85 weight-% powdered or liquid resin binder, and 0.1 to 15 weight-% titanium carbide particles. 